Polymer-anchored catalysts

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

The most frequently used organic supports are polystyrene and styrene-divinylbenzene copolymer beads with functional groups such as diphenylpho.sphine covalently bonded. The polymer-anchored catalyst complex can then be obtained, for example, by displacement of a ligand already co-ordinated to a soluble metal complex (Cornils and Herrmann, 1996) ... 

Sasson and Rempel [97] showed that the system [(PPh3)3RuCl2]/secondary alcohol is suitable for the selective transformation of 1,1,1,3-tetrachloro into 1,1,3-trichloro compounds. Similarly, Blum and coworkers [98, 99] employed [(PPh3)3RuCl2] as well as polystyrene-anchored Rh, Ru and Ir complexes for the hydrogen transfer from alcohols to trihalomethyl compounds, leading to dihalo-methyl derivatives. For example, one of the Cl atoms of 2,2,2-trichloro-l-phenyl-ethanol was displaced by H at 140-160 °C in 2-propanol. The polymer-anchored catalysts proved to be resistant to leaching [99]. 

The alternative strategy for heterogenization has been pursued by Blechert and co-workers, for a polymer-supported olefin metathesis catalyst. A polymer-anchored carbene precursor was prepared by coupling an alkoxide to a cross-linked polystyrene Merrifield-type resin. Subsequently, the desired polymer-bound carbene complex was formed by thermolytically induced elimination of ferf-butanol while heating the precursor resin in the presence of the desired transition metal fragment (Scheme 8.30). 

It has been shown that this reaction is also promoted by RhA(CO)2 (A = anthranilato). This finding led to the development of a reusable polymer-anchored Rh-catalyst for this process. 

The use of supported (i.e., heterogenized) homogeneous catalysts offers another possibility for easy catalyst separation. New examples include polymer-anchored Schiff-base complexes of Pd(TT),446 PdCl2(PhCN)2 supported on heterocyclic polyamides,447 various Pd complexes supported on crosslinked polymers 448 sol-gel-encapsulated Rh-quatemary ammonium ion-pair catalysts,449 and zwitterionic Rh(T) catalysts immobilized on silica with hydrogen bonding.450... 

Evidently, many simple chiral organic compounds that act as catalysts can be covalently bound to polymer backbones. Polymer-anchored quinine catalyzes the asymmetric Michael addition of a jS-keto ester to methyl vinyl ketone, which proceeds in 22-42% optical yield (Scheme 13) (29). 

The catalytic epoxidation proceeds via the formation of peroxytungstic acid. Similarly, other metal catalysts are effective in the H2O2 oxidation. Aqueous conditions are not appropriate for epoxidations since epoxides are prone to undergo acid-catalyzed hydrolysis36. Polymer-anchored catalysts are conveniently separated from the reaction mixture after catalyzed H2O2 epoxidations (equation 8)9. 

The determination of structure and bonding of polymer anchored catalysts is another area where the insolubility of the materials often precludes solution spectroscopic studies and one is limited to techniques that can be applied to irregular solids (57). In addition, combining oxygen plasma etching and surface analysis allows investigation of the depth of penetration of the metal into the polymer and allows detection of components that require concentration to allow detection. 

The polymer-anchored catalyst (jt-C3H5)Ru(CO)3X (X = Cl, or Br) on poly(4-vinyl-pyridine) has also been investigated, and has been shown to be an active catalyst for alkene isomerization (Figure 5.2) [31]. 

In other reactions, particularly where strongly complexing reactants, e. g., carbon monoxide, are involved, leaching of the immobilized metal center may take place. Generally, the parameters to be considered in a polymer-anchored metal complex catalyst are of a manifold nature. It is still an unsolved problem and an incompatible situation that, on the one hand, a leaching process should be avoided while, on the other hand, sufficient activity and the selectivity necessary for industrial applications are to be maintained. As a consequence it has become... 

The more often practiced routes to polymer-anchored complex catalysts include the displacement of a ligand already coordinated to a soluble metal complex by a polymer-bonded ligand [38] (eq. (5)), or the splitting of a weakly bridged dimeric metal complex [34] (eq. (6)). 

The selectivity of metal catalysts improves in some reactions with alloying for example the alumina-supported Pd-Cu catalyst hydrogenales butadiene to 1-butene with 99% selectivity, i.e. the isomerization is less than lOli. The explanation is that hydrogen adsorption decreased on the Cu-containing catalysts . Similarly, better selectivities were observed with a polymer anchored Pd, or a Pd-Co catalyst in the gas-phase hydrogenation of butadiene and cyclopentadiene in a hollow-liber reactor 2 in the liquid-phase hydrogenation of 1,5-hexadiene with Pd-Ag catalyst. ... 

A series of polymer-anchored epoxidation catalysts was obtained by modifying Merrifield resin with imidazole [61], diphosphines [62], or piperazine [63] followed by treatment with UV-activated Mo(CO)6. High activities in the epoxidation of cyclic (cyclooctene, cyclohexene, indene, and a-pinene) as well as linear alkenes (styrene, a-methylstyrene, 1-heptene, 1-dodecene, cis- and frans-stilbene) were observed using TBHP as oxidant. The catalysts were recovered and reused up to 10 times in the epoxidation of cyclooctene without loss of activity. 

Recently, a series of polymer-anchored tungsten carbonyl catalysts based on modified polystyrenes was prepared [86]. Polymer modification was carried out by reaction of chloromethylated polystyrene (2% cross-linked with DVB) with diphosphines, di- and triamines, pyrazine, 4,4 -bipyridine, and imidazole. The polymers were treated with W(CO)6(TEIF) and their catalytic performance was evaluated in the epoxidation of cyclooctene. Different solvents and oxidants were tested and epoxide yields up to 98% were obtained using the system CH3CN/ H2O2. A detailed catalyst recycling study was carried out and the catalyst containing 4,4 -bipyridine units kept constant activity over 10 reactions whereas other catalysts revealed deactivation. 

Since 2000 a few catalysts for asymmetric epoxidation based on polymer-anchored chiral l,l -bi-2-naphthol (BINOL) have been developed. Polystyrene-supported BINOL was prepared by radical copolymerization of styrene with BINOL, bearing 4-vinylbenzyloxy groups in the 3- or 6-position [96]. Immobilization of lanthanum or ytterbium was accomplished by treatment of the polymers... 

Systematic studies on the isomerization of W-allylamides 24 and -imides to aliphatic enamides 25 were carried out with iron, rhodium, and ruthenium complexes as catalysts, Eq. (8). Regrettably, no prochiral substrate was applied for the rhodium catalyst bearing polymer-anchored DIOP [33]. In the framework of a study on the conjugative interaction in the isomerization of 1-azabicyc-lo[3.2.2]non-2-ene 26 to orthogonal enamine 27, catalyzed by either f-BuOK or RuH(NO)(PPh3)3, the enamine formation was calculated to be favored by 4 kcal mob, Eq. (9) [34]. Recently, the palladium-catalyzed isomerization of the N-acyl-2,5-dihydropyrroles 28 to N-formyl-2,3-dihydropyrroles 29 was reported, Eq. (10) [35]. 

C. U. Pittman, Jr., S. E. Jacobson, L. R. Smith, W. Clements, H. Hiramoto, Polymer-Anchored Homogeneous Hydrogenation Catalysts and Their Use in Multistep Synthetic Reactions, in Catalysis in Organic Syntheses. P. N. Rylander, H. Greenfield, Eds., pp. 161-180, Academic Press, New York, 1976. 

R. D. Sanner, R. G. Austin, M. S. Wrighton, W. D. Honnick, C. U. Pittman, Jr., Photoactivation of Polymer-Anchored Catalysts. Iron Carbonyl Catalyzed Reactions of Alkenes, Chapter 2 in Interfacial Photoprocesses Energy Conversion and Synthesis, M. S. Wrighton, Ed., pp. 13-26, Advances in Chemistry Series 184, ACS Publishers, 1980. 

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